Semiconductor integrated circuits are produced by a plurality of processes in a wafer fabrication facility (fab). These processes, and associated fabrication tools, may include thermal oxidation, diffusion, ion implantation, RTP (rapid thermal processing), CVD (chemical vapor deposition), PVD (physical vapor deposition), epitaxy, etch, and photolithography. During the fabrication stages, semiconductor wafers are monitored for quality assurance and yield using various metrology tools.
Virtual metrology is a technique that forecasts results of a semiconductor process based on modeling and previously collected data (also referred to as training data). As semiconductor fabrication progresses to advanced technology node processes (e.g., 90 nm to 65 nm to 45 nm to 32 nm and beyond), practical implementation of virtual metrology becomes more constrained by online production needs, such as recipe changes, different process targets, and limited amount of available training data. Existing virtual metrology methods and systems typically focus on accuracy of the virtual metrology forecast results, rather than the practicality of the implementation of virtual metrology in semiconductor fabrication facilities.
Thus, while existing virtual metrology methods and systems are generally adequate for their intended purposes, they are not satisfactory in every aspect.